Uniaxial behavior of circular ultra-high-performance fiber-reinforced concrete columns confined by spiral reinforcement

This paper presents results from the uniaxial tests of six large-scale ultra-high-performance fiber-reinforced concrete (UHPFRC) circular columns confined by spirals. The UHPFRC used in this study had 1.5% of hybrid micro-steel fibers (1.0% of 19.5 mm fibers and 0.5% of 16.3 mm fibers) in the mixture and had compressive strengths varying from 163 to 181 MPa. The effects of the volumetric ratio of spiral reinforcement, compressive strength of concrete, and presence of hybrid micro-steel fibers on the axial load responses, including post-peak deformability, were investigated. In addition, the ductility level reached by circular UHPFRC columns designed according to the minimum spiral reinforcement of current design provisions of the CSA A23.3-14 Standard and the ACI 318-14 Code were evaluated to investigate the applicability of these equations to UHPFRC columns. Test results showed that the combined effect of the minimum spiral reinforcement and steel fibers resulted in sufficient post-peak ductility of the UHPFRC columns. To investigate the efficiency of the shape of the confinement reinforcement, the test results of the circular UHPFRC columns confined by spirals were compared with those from equivalent-sized square UHPFRC columns confined by hoops. The test results demonstrate the superior performance of circular spirals for developing the ductile behavior of UHPFRC columns than the same volumetric ratio of rectilinear hoops. A design recommendation for spiral reinforcement that ensures the ductile behavior of UHPFRC columns in moderate seismic regions is proposed.